Ion source for mass spectrometers employing means for flattening equipotentials within the ion production region



Aug. 19, 1969 w BROWN 3,462,595

ION SOURCE FOR MASS SIEC'IROMETERS EMPLOYING MEANS FOR FLATTENING EQUIPOTBN'IIALS WITHIN THE ION PRODUCTION REGION Filed Nov. 30. 1966 3 Sheets-Sheet 1 H Wonmcw I 3 4 8 TORH1G2- 1g E ELII IKTIIIII] \V V INVENTOR F'GJ BY HARM N W.BROWN l a RNEY Aug. 19, 1969 1 H. w. BROWN ION SOURCE FOR MASS SPECTROMETERS EMYLOYING MEANS FOR FLATTENING EQUIPOTENTIALS WITHIN THE ION PRODUCTION REGION Filed Nov. 50. 1966 3 Sheets-Sheet 2 2 a 2 H n W 2 l 4 m n i 6 Z 2 n 13 7 n v 3k f m w I I 2 N W "a 1 F H M 3 w H I V I WM At r VmWI VWAV WV HHH H m U 11 WW F 00 4 0+ 5 3 w H U a x M 1% i/ 4 Iv 2 w w i w; @r G 5 5 H T.

VARIABLE VOLTAGE SUPPLY INVENTQR. HARM ON w. BROWN My 1 a&.

ATTORNEY Aug. 19, 1969 H. w. BROWN 3,462,595

ION SOURCE FOR MASS SFEC'I'ROMETERS I'ZMPLOYING MEANS FOR FLATTENTNG EQUIPOTENTIALS WITHIN THE ION PRODUCTION REGION Filed Nov. 30. 1966 3 Sheets-Sheet 3 V-AV I 28 0 28 l 0 v V-AV v 1 45 i -25 45 1 I 25 B- x +51 \L V= /2 V Z i 349% v+Av u V+AV FIG 6A FIG.6B

INVENTOR.

' ATTORNEY United States Patent [1.5. Cl. 25041.9 5 Claims ABSTRACT OF THE DISCLOSURE The present invention relates in general to ion sources for mass spectrometers and, more particularly, to an improved ion source employing method and apparatus for adjusting the flatness of the ion accelerating field equipotentials within the ionizing beam path, whereby, when adjusted for optimum flatness, all the ions of the beam have a uniform beam potential.

Such an improved ion source lessens the energy spread of ions of like kind within the beam and thereby lessens the requirements of homogeneity of the analyzing fields in the mass analyzer section of the mass spectrometer for a given resolution of the output mass spectral data. Moreover, the more uniform ionizing potential, obtained by the improved ion source, permits useful mass spectral data to be obtained at ionizing electron beam energies low enough to avoid fragmentation of the molecules under analysis, thereby greatly facilitating interpretation of the output mass spectral data.

Heretofore, ion sources have been built which ionized and/or dissociated the gaseous material to be analyzed in a relatively gas tight chamber and within a region thereof of relatively intense uniform electric field for accelerating the ions into the beam. The ionizing and/or dissociating electrons entered the ion production region in the plane of the accelerating field equipotentials. In such a source, the ion accelerating electric field can have an intensity on the order of 1000 volts per inch. Such an ion source is described and claimed in copending U.S. patent application Ser. No. 534,857, filed lMar. 16, 1966, now US. Patent 3,443,088, issued May 6, 1969, and assigned to the same assignee as the present invention.

While such an ion source has a relatively high efiiciency, problems have been encountered in manufacture thereof. More specifically, if the ionizing electron beam is displaced slightly, as for example 0.001", from the exact plane of mechanical symmetry of the ionizing chamber, there is introduced a 1 volt error in the ionizing electron energy. In practice it has been found very difficult to maintain manufacturing tolerances of the assembled source to such a high degree. Moreover, if the potential applied to the accelerating electrode, repeller electrode and side wall of the ionizing chamber depart slightly from an optimum relationship then the accelerating equipotentials cross over the ionizing electron beam path, causing ions to be produced at different energies. This energy spread of the ions makes it diflicult to determine the ionizing potential for gases being analyzed and if the energy spread is sufliciently large may produce unwanted fragmentation of the gas under analysis, introducing additional mass peaks and, thereby, making interpretation of the output mass spectral data more difficult.

In the present invention, the ion source includes means for adjusting the flatness and positions of the accelerating field equipotentials over the ion production region of the ion source, whereby the ions are produced at a uniform 3,462,595 Patented Aug. 19, 1969 ionizing potential, thereby lessening the energy spread of ions of like kind within the beam. Reducing the energy spread of the ion beam lessens the requirements on the uniformity of the analyzing electric and magnetic fields of the mass analyzer section of the mass spectrometer for a given output resolution. Moreover, lessening the energy spread of the ions permits a more precise determination of the ionization potential and permits analysis of gaseous constituents without producing unwanted fragmentation of the gases under analysis.

In a preferred embodiment of the present invention, the equipotentials are adjusted for maximum flatness by means for adjusting the electrical potential applied to the central portion of the ionizing chamber relative to the operating electrical potentials applied to the ion accelerating and ion repeller electrodes of the ion source, whereby an extremely simple and effective control over the ion energies is obtained.

The principal object of the present invention is the provision of an improved ion source for mass spectrometers.

One feature of the present invention is the provision of method and apparatus for adjusting the flatness of ion accelerating electric field potentials over the ion production region of an ion source, whereby essentially all the ions of the beam can be produced at a uniform potential.

Another feature of the present invention is the same as the preceding feature wherein the ion source includes an ionizing chamber having an ion accelerating electrode and an ion repeller electrode separated by a hollow electrode surrounding the ion production region and being determinative of the ionizing potential, and wherein the means for adjusting the flatness of equipotentials of the ion accelerating field comprises an adjustable electrical circuit for adjusting the ratio of the potential of the hollow electrode to the potential difference between the other two electrodes, whereby an extremely simple and effective control over the uniformity of the ionizing potential is obtained.

Another feature of the present invention is the same as any one or more of the preceding features in combination with a cycloidal mass spectrometer, whereby the requirements of uniformity of the electric and magnetic fields of the cycloidal mass analyzer section of the spectrometer are lessened for a given mass resolution.

Other features and advantages of the present invention will become apparent upon a perusal of the following specifications taken in connection with the accompanying drawings wherein:

FIG. 1 is a schematic drawing of a cycloidal mass spectrometer system employing features of the present invention,

FIG. 2 is a circuit diagram of the network for applying operating potentials to the electric field ion analyzer electrode array of FIG. 1,

FIG. 3 is an enlarged sectional view, partly schematic, of the ion source structure of FIG. 1 taken along line 33 in the direction of the arrows,

FIG. 4 is a view of the structure of FIG. 3 taken along line 4-4 in the direction of the arrows,

FIG. 5 is a plot of detected ion current for N and N+ versus electron volts of the ionizing electron beam for the ion source of FIG. 3, and

FIGS. 6A-6E are schematic diagrams depicting the potentials of the ion source and ion beam for various conditions of electrode asymmetry and electrical asymmetry of the ion source.

Referring now to FIGS. 1 and 2 there is shown a cycloidal mass spectrometer system. More particularly, an array of generally rectangular shaped ring electrodes 1 are insulatively supported Within a thin rectangular vacuum envelope 2, only partially shown, from a heavy rectangular flange, not shown, which closes off one end of the vacuum envelope.

The separate rings 1 of the electrode array are operated at slightly different electric potentials derived from a voltage source 3 via leads 4 connected at nodes 5 of a voltage divider network 6. The different potentials, applied to the different rings 1, established a region of uniform electric field E in the hollow interior of the ring electrode array. The electric field E is directed parallel to the line of development of the ring electrode array.

The electrode array is immersed in a uniform region of magnetic field H directed at right angles to the direction of the electric field E. The field H is conveniently produced by an electromagnet 7 with the vacuum envelope 2 being disposed in the narrow gap defined between a pair of pole pieces 8 of the magnet 7.

The envelope 2 is evacuated, in use, via pump 10 to a suitably low pressure as of 10- torrs. Gas to be analyzed by the analyzer section, including the array of electrodes 1, is introduced from a source 9 into the analyzer section through the vacuum envelope 2 via an inlet tubing 11 as of stainless steel. The inlet tubing 11 feeds gas at a desired rate into an ion source 12. The ion source 12 ionizes the gas and projects it through a slot into the crossed magnetic field H and electric field E of the analyzer.

Under the influence of the crossed electric and magnetic fields the ions are caused to execute cycloidal trajectories. However, only ions of a certain mass number, for given intensity of E and H, will be focused at a detector slot 13 located a certain focal distance from the source 12 and at the same electric potential. An ion detector 14 is positioned behind the slot 13 to produce an output corresponding to number of ions under analysis having the certain predetermined focused mass number, if any.

The output is fed to an amplifier 15 which amplifies the detected signal and feeds it to the Y axis of an X-Y recorder 16 wherein it is recorded as a function of a scan of the magnetic field intensity H produced by a scan generator 17. The output of the recorder 16 is a mass spectrum of the sample under analysis.

The ion source 12 (see (FIGS. 3 and 4) includes a metallic ionizing chamber 21 as of stainless steel which may be rhodium plated to reduce corrosion and contamination and within which gas to be analyzed is ionized and formed into a beam 22. The ionizing chamber 21 is segmented and separated by thin insulating sheets 23 as of 0.005" thick mica to provide three separate electrodes 24, 25 and 26. The center electrode 25 includes a hollow cylindrical bore as of 0.250" in diameter and 0.116" in axial length defining the central portion of the ionizing chamber 21. The ends of the ionizing chamber 21 are closed off by transverse walls 27 and 28 forming portions of electrodes 24 and 26, respectively. End wall 27 is centrally apertured to form a gas inlet passageway 29 in gas communication with an insulating section of the gas inlet pipe 11 for the introducing gas, to be analyzed, into the ion source 12. The opposite end wall 28 includes an ion beam exit slit 31 formed by a pair of slightly spaced apart knife edge plates 32 as of stainless steel sealed over a cylindrical bore 34 centrally located at the end wall 28. Bore 34 is, for example, 0.200 in diameter and the beam exit slit 31 is approximately 0.001 to 0.0004" in width as defined by the spacing between the plates 32. The elongated axis of the ion beam exit slit 31 is parallel to the direction of the magnetic field H which threads through the ion source 12 and ion analyzer rings 1. The gas inlet end wall 27 is counterbored at 35 to provide mechanical symmetry with the bore 34 in the ion beam exit wall 28.

A pair of cylindrical electron beam passageways 36, axially aligned with the direction of the magnetic field H, and as of, for example, 0.040" in diameter pass through the inner wall of the center electrode 25. The passageways 36 define an electron beam path 37 therebetween coinciding with and lying within the transverse structural plane of symmetry of the ionizing chamber 21. A filamentary thermionic emitter 38 is axially aligned with the beam passageways 36 for projecting a beam of electrons across the ionizing chamber over the beam path 37. The emitter 38 is heated by a current drawn from a battery 39. The central electrode 25 serves as the anode for the emitter 38 and the anode potential for the emitter 38 is supplied from a variable voltage supply 41 connected between the filament 38 and its anode 25. The electron beam 37 serves to ionize and/or to dissociate gas particles within the electron beam path 37 inside the ionizing chamber 21 and is collected by a metallic collector electrode 40 operating at anode potential and covering over the beam exit hole 36.

Electrode 24 serves as the repeller electrode for the ion source 12 and is supplied with its independent operating electrical potential U as of 160-200 volts from a variable voltage source 42. Electrode 26 serves as the ion beam accelerating electrode and is preferably operated at ground potential.

The intermediate hollow electrode 25 serves to produce a region of uniform intense electric field E, as of more than 150 volts/cm. and preferably about volts/ inch, over the central ion production region 43 of the beam path 37 defined by the shaded region of the drawing. The central electrode 25 is preferably operated at a potential midway between the operating potentials applied to electrodes 24 and 26. The operating potential for the central electrode 25 is derived from an adjustable voltage dividing network 45 formed by resistors 46 and 47 as of 10 K9 each and/or potentiometer 48 connected across the voltage supply 42.

In operation, gas to be analyzed by the cycloidal mass spectrometer is introduced into the ion source 12 via gas inlet pipe 11, 11' and inlet passageway 29. The gas is ionized by the electron beam in the beam path 37. Under the influence of the uniform electric field E produced by the system of electrodes 24, 25 and 26, the ions within the central beam path region 43 are rapidly swept through the ion beam exit slit 31 to form a well defined ribbon ion beam 22 emerging from the exit slit 31. The central ring shaped electrode 25, operated at a potential midway between the repeller and exit electrode potentials and placed in a position of structural symmetry, allows the ionizing region 43 to be placed in a position of optimum electric field uniformity. By making the inside diameter of ring 25 larger than the axial length, the intensity of uniform electric field is made relatively large as of greater than volts/ cm. averaged over the ionizing region 43. Thus, ions produced are rapidly withdrawn through the exit slit 31. As a result, the ion source 12 yielded a sensitivity of 2 10 amps/torr with exit and detector slits of the aforementioned dimensions giving a detected mass resolution greater than 1000 between half amplitude points on the detected mass peak.

Passing the ionizing electron beam path through the chamber 21 in a plane of electrical symmetry with the electrons directed parallel to the equipotentials of the uniform electric field E yields substantially improved definition of the ionizing and dissociation potentials of the ion source 12. For example, nitrogen gas introduced into the ionizing chamber 21 may undergo either one of the following reactions:

The first reaction (1) results in only ionizing the nitrogen gas to produce N +ions with mass number 28. While monitoring this mass number on the mass spectrometer and decreasing the ionizing electron beam anode voltage, a plot of ion current versus ionizing electron volts is obtained as shown in FIG. 5. The point where the mass 28 ion goes to zero represents the ionizing potential in electron volts for the nitrogen gas under analysis. This is of importance to chemists and it is desired that this point be well defined. The ion source of the present invention permits good resolution of ionizing potential.

The second reaction (2) represents dissociation of the nitrogen gas molecule and the potential at which this occurs is of interest to chemists and, therefore, should be well defined. This potential is measured in the same way as the ionizing potential, only mass 14 is monitored instead of mass 28. The ion source 12 provides a well defined value for this potential as well.

It has been found that the uniformity of the potenial of the ion accelerating field over the ion production region 43 is critically dependent upon the mechanical and electrical symmetry of the ion source and upon the precise alignment of the beam 37 with the plane of electrical symmetry. More specifically, the ion accelerating electric field in the ion production region is on the order of 1000 volts/inch. This means that if the beam path 37 is displaced by as little as 0.00l" from the flat equipotential surface, at hollow electrode potential, which extends across the central portion of the hollow electrode 25, that there will be a spread in potential at which the ions of the beam are produced by plus or minus approximately one volt. This is a relatively tight tolerance to be maintained in manufacture. In addition, this same type of spread in ionizing potential is obtained, even if there is perfect mechanical symmetry, if the hollow center electrode is not operated at precisely a voltage equal to one half the voltage between the accelerating electrode potential and the repeller electrode potential.

As previously mentioned, this spread in the ionizing potential makes it difiicult to precisely determine the ionizing potential of a given gas, can produce unwanted fragmentation of gases under analysis, and reduces the resolution of the output mass spectra for a given homogeneity of the electric and magnetic fields of the mass analyzer section.

Thus, in the present invention, means, preferably electrical, are provided for adjusting the accelerating field equipotential surfaces relative to the ion production region such that the beam ions are produced at a uniform potential. The adjusting means can be a mechanical adjustment for physically moving the axial position of the beam path 37 within the hollow electrode. Such a device may comprise a dielectric adjusting screw 51 and nut. Alternatively, the mechanical adjustment may comprise dielectric adjusting screws 52 and nuts for adjusting the axial positions of the end walls 27 and 28 relative to the central hollow electrode 25 such as to flatten the equipotential over the ion production region. However, a preferred embodiment employs any one or more of the adjustable resistors 46, 47 or potentiometer 48 of the potential divider network 45 for flattening the equipotentials within the ion production region 43.

Referring now to FIGS. 6A-6E, there are shown a number of sketches depicting how lack of mechanical and/or electricalsymmetry of the ion source can cause the ions to be produced at non-uniform potentials and how the equipotentials can be corrected such that the ion beam is produced at a uniform potential. The curvatures and asymmetries of the equipotential surfaces are exaggerated for the sake of explanation.

FIG. 6A shows the desired situation. The ion production region 43 lies on the flat equipotential surface V where V: U/2, where U is the potential difference between the accelerating electrode 28 and the repeller electrode 27. This condition exists when the source has perfect mechanical and electrical symmetry with respect to the ion production, region 43. The potential V of the ion beam is depicted as a function of the distance across the ribbon beam and is shown as a straight horizontal line at a value of U/ 2.

FIG. 6B depicts a condition of mechanical symmetry and electrical asymmetry, in that the hollow electrode 25 is operating at a voltage V U/2. For such a case, it is seen that the equipotential VAV dips through the ion production region 43 and that the desired flat equipotential surface V is below the ion production region 43. The beam has a non-uniform potential as shown. This condition can be corrected by moving the electron gun 38 down to the V plane, by decreasing the voltage on the hollow electrode 25, as by adjusting one or more of resistors 46, 47 or potentiometer 48, by moving the repeller electrode 27 away from the ion production region 43, or by moving the accelerating electrode 28 closer to the ion production region 43.

FIG. 6C shows a condition of electrical asymmetry in that the hollow electrode is operating at less than U/2 volts. This condition is correctable by reversing any one or more of the adjustments offered for correcting the ,condition shown in FIG. 6B.

FIG. 6D shows a condition of mechanical asymmetry in that the hollow electrode 25 is closer to the repeller electrode 27 than to the accelerating electrode 28. This condition is correctable by, increasing the voltage on the hollow electrode 25 to something greater than U/2, by moving the repeller electrode 27 away from or the accelerating electrode 28 toward the ion production region 43, or by moving the ion production region 43 up to the V equipotential surface.

FIG. 6E shows a condition of mechanical asymmetry in that the hollow electrode 25 is closer to the accelerating electrode 28 than to the repeller electrode 27. This condition is correctable by reversing any one or more of the corrective adjustments offered for the condition depicted in FIG. 6D.

One method for making the aforementioned adjustments and especially the electrical adjustments is to introduce a known gas such as toluene C H into the ion source. The electron beam voltage of voltage supply 41, used for ionizing the gas, is set for 9 volts. The ionizing potential for toluene is 8.82 volts and the first fragment voltage is 11.80 volts. The mass spectral lines for toluene and the first fragment of toluene, namely, C H occur at 92 and 91 a.m.u., respectively. The potential applied to the center electrode 25 is then adjusted by adjusting one or more of the resistors 46, 47 or the pickolf of potentiometer 48 until the toluene peak at 92 a.m.u. remains but the fragment peak of 91 a.m.u. disappears. The adjustment is then continued in the same sense until the toluene peak at 92 a.m.u. begins to be reduced. Thus, when the potential on the center electrode 25 is set as aforedescribed, the potential over the ionizing region 43 is very uniform, i.e., within a volt or less of the electron beam voltage.

The ion source 12 should not be wasteful of gas to be analyzed, as unnecessary leaks in the ionizing chamber produce wasting of the sample and contamination of the spectrometer. In the ion source 12 of the present invention, with a beam exit slit 31 of the aforementioned dimensions, the chamber 21 was free of unnecessary leaks to the extent that the total leak rate taken through the source 12 from the inlet 29 for all openings, including the beam exit slit, was less than 2 liters/second for nitrogen gas.

The ion source 12 has been described as it would be used to produce a positive ion beam. However, the source is equally useful for producing negative ion beams by merely reversing the terminals of the voltage supply 42. The negative ion beam would be analyzed by reversing the direction of the magnetic field H, and the direction of electric field E.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a mass spectrometer apparatus; means forming an ion source for producing a beam of ions to be mass analyzed; said ion source including, means forming an accelerating electrode having an opening therein through which the beam of ions is projected, means forming an ion repeller electrode spaced from said accelerating electrode for repelling ions therefrom toward said accelerating electrode, means forming a hollow electrode structure disposed between said ion accelerating and ion repelling electrodes and containing therewithin an ion production region of said ion source, said hollow electrode structure having a longitudinally directed bore therethrough to define the hollow portion of said electrode, said bore having symmetrical cross sectional dimensions about a plane transverse to the longitudinal axis of the longitudinal bore and disposed substantially midway along the length of said bore, said accelerating, said repeller and said hollow electrode defining a generally closed ionizing chamber bounded on the ends by said accelerating and repeller electrodes and bounded on its sides by said hollow electrodes, means for applying difierent operating electrical potentials to said electrodes with the potential applied to said hollow electrode being intermediate the potentials applied to said accelerating and repelling electrodes to establish a series of ion accelerating field equipotential surfaces traversing the central region of said hollow electrode, means for projecting a beam of ionizing electrons over a predetermined beam path extending across said hollow electrode generally parallel to the equipotential surfaces of the ion accelerating electric field and substantially within a plane of structural symmetry inside of said ionizing chamber to define said ion production region of said ion source, and means for adjusting the flatness of the equipotential surfaces of the accelerating field in said ion production region, whereby a proper adjustment of said adjusting means permits the ions of the beam to be produced in a uniform region of ion accelerating field.

2. The apparatus of claim 1 wherein the applied operating potentials are of such a magnitude combined with the dimensions of said ionizing chamber to produce a central ion production region traversed by an average ion accelerating electric field intensity greater than volts/cm.

3. The apparatus of claim 1 wherein said means for adjusting the flatness of the equipotential surfaces comprises means for adjusting the ratio of the operating potential of said hollow electrode to the potential differ- ,ence between said accelerating and repeller electrodes, whereby an extremely simple and effective control over the uniformity of the ion accelerating field in said ion production region is obtained.

4. The apparatus of claim 3 wherein said means for adjusting the ratio of potentials includes, a potentiometer connected in circuit between said accelerating and repeller electrodes, and said potentiometer including an adjustable pickoff electrode connected to said hollow electrode structure for supplying its operating potential.

5. The apparatus of claim 1 in combination with a cycloidal mass spectrometer for mass analyzing the ion beam produced by said ion source.

References Cited UNITED STATES PATENTS 2,780,729 2/ 1957 Robinson et al. 3,155,826 11/1964 Peters. 3,355,587 ll/l967 Ienckel.

RALPH G. NILSON, Primary Examiner A. L. BIRCH, Assistant Examiner 

